If you don't know what a Mary Sue (MS) is, I'll give you my version and then point you toward some other resources for more in-depth reading. To me, a MS is a character, usually the main character, that is inexplicably awesome, stuff always goes right for them, and if it doesn't they are usually prophesied/fated to "win" at the end of the story. Their enemies lose, big time. And they get the girl/guy/unicorn/whatever. They might be remarkably similar to the author, probably a very idealized version of her or him. TV Tropes has a great article on the MS. Here's one from Know Your Meme and another from Fanlore.